A broad variety of new demands are being placed on the electromagnetic spectrum, leading to difficulty in allocating radio wave frequency bands as new kinds of equipment based on radio wave communication are developed. Several recent examples of such developments include extremely light-weight, hand-portable cellular telephones, wireless local area networks for linking computer systems within office buildings in a readily reconfigurable fashion, wireless infant monitors, wristwatch-sized paging apparatus, and a host of other devices for promoting rapid, efficient, and flexible voice and data communication.
This has resulted in pressure to employ progressively higher radio frequencies (e.g., &gt;500 MegaHertz) and need to utilize spectral space more efficiently. These trends create requirements for frequency selection components capable of high frequency operation and increasingly narrow passbands. Additionally needed are low insertion loss coupled with improved out-of-band signal rejection, in a small form factor and with low power consumption.
To meet these demands, there is at the present time much effort and expense going into research and development relating to acoustic wave devices such as filters, delay lines, resonator devices and lattice filters for a variety of practical applications. Acoustic wave devices are becoming particularly important in the construction of electronic signal processing equipment, such as radios, paging devices, and other high frequency electronic apparatus, because they can be readily constructed on planar surfaces using integrated circuit fabrication techniques, are robust and compact, require no initial or periodic adjustment, and consume no static power.
These devices operate by conversion of electrical signal energy into acoustic energy, which then propagates through or in the near surface region of a suitable acoustic medium. This energy conversion process often relies on the piezoelectric effect as manifested by materials which form certain non-centrosymmetric crystal types, such as ZnO, CdTe, LiNbO.sub.3, LiTaO.sub.3, SiO.sub.2, BiGeO.sub.20, GaAs, and the like. Acoustic energy is usually converted back to electrical energy by another or the originating transducer, producing the desired electronic performance. A basic equation describing signal frequency f.sub.sig, acoustic wavelength .lambda., and properties of the acoustic medium is: EQU .lambda.f.sub.sig =v.sub.s ( 1)
where v.sub.s represents acoustic velocity in the acoustic medium. For a given acoustic velocity v.sub.s, increased f.sub.sig requires reduced .lambda..
Wave propagating acoustic transducers rely on electrodes which are usually a fraction of a wavelength in width. Photolithographic constraints together with Eq. 1 determine an upper frequency limit by setting a lower electrode width limit. The current minimum electrode width is about one micrometer for practical mass-production equipment and techniques. This minimum electrode width sets the upper frequency limit between about one and two GigaHertz. At present, this is a frequency range of intense interest for development of new electronic products.
The material used for the transducer electrodes typically has either mass density or stiffness mismatch to the acoustic impedance of the acoustic medium. This often results in acoustic reflections from electrode structures. Examples of dense materials providing mass mismatch include gold, silver, osmium, and the like. Materials which are extremely stiff, and so produce stiffness mismatch, include chromium and tungsten.
Further, the mere presence of conductive materials engenders acoustic impedance mismatch through the piezoelectric effect, resulting in acoustic reflections. Materials such as aluminum and alloys thereof, which are flexible, have low density, are excellent electrical conductors, and which are easily prepared and patterned in thin film form, are preferred for such acoustic device electrodes.
Control of fabrication variables, such as the ratio of electrode width to spacing, metal thickness, and the like, becomes progressively more difficult as photolithographic limits are approached, i.e., as the desired electrode widths become smaller. This results in reduced fabrication yields for devices requiring electrode widths at or near the photolithographic and etching limits.
What is needed are techniques for device realization which are insensitive to manufacturing variations, which do not result in significant compromise of device capabilities or performance, and which are easily implemented in a fashion consistent with current acoustic device design, fabrication and use practices.